Carbon-14 has a half-life of 5,730 years. How long will it take for 112.5 g of a 120.0 g sample to decay radioactively?
The half life of a substance is the time taken for the amount of a substance present to halve. If 112.5g has decayed we have 7.5g left. To get to 7.5g we need to halve 120g four times.
The total time elapsed in this time will be four times the half life, so